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The new field of gravitational wave astronomy is deepening our understanding of how paired black holes form, writes Andrew Masterson.

The detection of gravitational waves has launched “an entirely new subfield of astronomy” that will eventually explain how black holes join up into pairs.

That’s the prediction of Steinn Sigurðsson from Penn State University in the US, commenting in the journal Nature on research examining three gravitational waves emanating from binary black hole systems.

If the black holes had previously existed independently before merging, the theory suggests, then the distribution of the measured spin should be “isotropic”. That is, the spins of each black should be aligned at random, with no connection to the direction of their orbit around each other.

If, on the other hand, the black holes arose from the death of an already paired star system, then the spin should be preferentially aligned with the orbit.

Farr and colleagues report that on the analysis of data gleaned from gravitational wave detections associated with the black hole detections dubbed GW150914, LVT151012, GW151226 and GW170104, the odds are very slightly in favour of isotropic results – indicating that pairs are created when individual black holes effectively collide with each other.

Available data, however, is not sufficient to draw firm conclusions. However, the study authors and Sigurðsson agree that real benefit of the research lies in the precision of the analysis and the consequent reduction of the amount of new evidence required before a definitive conclusion can be made.

Farr’s team estimate that only another 10 gravitational wave detections associated with binary systems will be needed. At that point, the authors say, “the existing preference for either an isotropic spin distribution or low spin magnitudes for the observed systems will be confirmed (or overturned) confidently in the near future”.

Sigurðsson adds that the findings are “important because they tell us how many data are needed to test the main formation theories, and show that the number of required observations is likely to be achieved in the near future.”